FIG. 1 illustrates a 2-pole discharge element in the prior art, and the element includes discharge electrode 1 and discharge electrode 2 at both ends of a cylindrical tube made of a ceramic insulator, and a discharge gap is formed inside the tube, and it has a structure filled with a discharge-assisting material (gas) inside the discharge gap.
In a discharge element as described above, when high voltage is applied between discharge electrode 1 and discharge electrode 2, a discharge-assisting material filled in the discharge gap starts a glow discharge while being ionized, and immediately it is followed by an ark discharge when a discharge current becomes large by the glow discharge, and thus a voltage applied between the discharge electrodes is instantaneously discharged and vanished.
FIG. 2 illustrates a 3-pole discharge element in the prior art, and the element includes earth electrode contacted with discharge-assisting material (gas), discharge electrode 1 and discharge electrode 2 at both ends of a cylindrical tube made of a ceramic insulator, and a discharge gap is formed by discharge electrode 1 and discharge electrode 2, and it has a structure filled with a discharge-assisting material (gas) inside the discharge gap.
In a 3-pole discharge element of FIG. 2, when high voltage is applied between discharge electrode 1−discharge electrode 2, discharge electrode 1−earth electrode, or discharge electrode 2−earth electrode, a discharge-assisting material filled therein starts a glow discharge while being ionized, and immediately it is followed by an ark discharge when a discharge current becomes large by the glow discharge, and thus a high voltage applied between the electrodes is instantaneously discharged and vanished.
As seen in FIGS. 1 and 2, in a conventional discharge element, all of electrodes constituting the discharge element are physically and electrically connected to discharge-assisting material filled therein.
The discharge element is a gas-filled relay tube in which the discharge-assisting material is gas or vacuum, and it has a discharge characteristic that the tube is discharged at a level of about 90 V against direct current or transient voltage having a slow rising speed, such as a level of 100 V/sec. However, when a fast transient voltage, such as a level of 1,000 V/μs, is applied, it has a discharge characteristic that the tube is not discharged at a level of 700 V or lower.
On the basis of the discharge characteristic of a convention discharge element, the recommendation of ITU-T is a different regulation from that of ANSI/IEEE. For a discharge element used as a protection element of PSTN lines, the ITU-T recommends that the element should be discharged at a level of 600V or lower against a slow rising speed, such as 100 V/sec while regulations such as ANSI/IEEE 61000-4-5 and UL497 define a fast transient characteristic of 1.2 μs/50 μs, and therefore those regulations have a problem that cannot be compromised even among such international regulations.
In a state of disorder that even international regulations for such fast applied transient voltages are not unified, it is reality that the discharge element firmly occupies its place as a surge protection element in the communication field.
As an example, a UL-certified discharge element of EPCOS, 3P230-05, is discharged at 225 V for direct current, but is discharged at 850 V as a result of testing a fast transient waveform with IEC C62.41 standard.
Accordingly, for a testing according to international regulations that protection elements based on PSTN should be discharged within 600 V in the ITU-T, discharge elements typically used against a characteristic of transient voltage which is quickly applied, such as an induced surge, are all inadequate, and it is reality that lightning damage cannot be prevented even when a terminal box or MDF protection plug is actually installed in a building.
Although the discharge-type element is universally used as a protection element for general communication in RS-232, 422, 485, or the like as well as in the PSTN field, efforts for reducing residual voltage after discharge have been made by adding a multi-level protection circuit, such as double or triple protection, due to the limit of a discharge characteristic thereof.